Fused coupler technology, wherein optical fibers are bundled together, heated, and pulled lengthwise, is commonly used to produce couplers, combiners and splitters for optical communication systems, medical devices and other industrial applications. Generally, a combiner is a passive fiber optic coupler in which power from several input fibers is combined into one output fiber. Conversely, a splitter divides light from a single input fiber into two or more output fibers. The coupler represents the general case where inputs from one or more fibers are mixed and distributed among one or more output fibers.
Combiners in particular are seeing new applications for diode pumped lasers, including fiber lasers and solid-state lasers, and diode pumped optical amplifiers. They are used to combine multimode optical pump power from a multiple sources, such as multimode laser diodes, and transfer the combined pump power into the inner cladding of a multiclad fiber or into a multimode fiber. A multiclad fiber typically has a small core (that typically transmits a singlemode or a small number of modes) surrounded by an inner cladding layer of lower refractive index and significantly larger cross-section that transmits the multimode pump power. An outer cladding of even lower refractive index causes the multimode pump power to be confined in the inner cladding by total internal reflection. The multiclad fiber is used to combine a singlemode (or multimode mode) signal in the core, along with multimode pump power in the inner cladding, to a separate device which may be used for amplification. These mode multiplexing combiners are typically used with cladding-pumped fibers. Cladding-pumped fibers are a special case of multiclad fiber where the multimode light propagates within the core and inner cladding interacting with special dopants (such as rare-earth elements like Er) in the core that absorb the pump photons and radiate photons at a different wavelength. Under suitable conditions, the special dopants in the core cause stimulated or spontaneous emission at the different wavelength and can operate in the form of a fiber laser or optical amplifier. Multiclad fibers containing special dopants for the purpose of lasing or amplification are known as cladding-pumped fibers.
For any coupler, splitter, or combiner, it is desirable to maximize the throughput of optical power from any input fiber, through the device, and through any output fiber. For convenience, the case of a combiner is further described, recognizing that the same principles apply equally to splitters and couplers. The throughput depends on efficiently transferring the total brightness from all the input fibers into a single output fiber having sufficient capacity to carry the combined brightness. This transfer can be analysed using modal analysis, ray-tracing methods, or by simple matching of input and output brightness. Conservation of brightness is based on the LaGrange Invariant of an optical system and is typically characterized by the quantity etendue, which is the product of the area of illumination times the extended solid angle. For a fixed level of optical power, increased brightness implies a decrease in etendue. For a step index multimode optical fiber, the etendue can be approximated by E=π2/4 NA2D2. If such a step index fiber is tapered, its etendue remains constant while the effective numeric aperture (NA) increases as the diameter (D) decreases. In this analysis, the NA refers to the maximum angle of light entering or exiting the optical fiber according to NA=sin(acceptance angle).
In order to efficiently transfer power between two optical elements (in this case from an input fiber bundle into an output fiber), two requirements must be satisfied. First, the etendue on the input side should be less than or equal to the etendue on the output side, otherwise the coupling efficiency will be limited by Eout/Ein. Second, the areas must be matched at the junction.
Prior art combiners, such as that described in U.S. Pat. No. 5,864,644 to DiGiovanni et al., rely on the tapering process to eliminate interstitial voids between input fibers, and to develop a suitable circular cross-section in the input fiber bundle. However, it is not possible to solely rely on the tapering process to achieve the requirements for low loss combiners, especially when there is a range in the number of input ports, or if the output fiber is non-circular.
It is, therefore, desirable to provide an improved optical coupling device that substantially eliminates interstitial spacing between input fibers, while providing a good cross-sectional match between the input fiber bundle and the output fiber independent of the number of fibers bundled together.